The present invention relates generally to shielding of electromagnetic interference (EMI) and radiofrequency interference (RFI). More specifically, the present invention relates to metallization and grounding of electrical cables and connectors to provide electromagnetic shielding from electromagnetic interference, radiofrequency interference, and electrostatic discharge (ESD). As subsequently used herein, xe2x80x9cEMIxe2x80x9d shall include ESD, RFI, and any other type of electromagnetic emission or effect.
Cables and connectors must be allowed to deliver their signals unimpeded. Unfortunately, cables and connectors for connecting electronic devices and specialized cabling that incorporates passive and active electrical devices in a flexible substrate material (e.g., flexible circuits) are both receptors and emitters of EMI radiation. Impingement of EMI can disrupt the functionality of the cable and connectors, and in some cases may cause electronic failure of the cables. With microprocessor speeds continuing to increase, the creation of EMI is a substantial concern to designers, manufacturers, and owners of electronic equipment.
Conventional cable shielding solutions include flexible conductive braiding, conductive epoxies, and conductive foils or tapes that can be wrapped around the dielectric cladding of the cable to provide shielding. Unfortunately, each of the conventional solutions have various drawbacks. For example, the conductive braiding is costly, the conductive epoxies are also costly and difficult to apply to the cladding, and the conductive foils and tapes must manually be wrapped around the cable body.
A particular problem of convention shielding solutions is leakage at the joint where the cable body shielding and connector attach. Gaps or xe2x80x9cslot antennasxe2x80x9d at joints or seams that break the continuous nature of the shield is a primary reason why shielding effectiveness degrades.
Current shielded cable solutions can provide shielding effectiveness in the range of 20 dB to 50 dB. Unfortunately, with the higher-speed microprocessor technology that is presently in use (and that is being developed) there is a need to provide consistent integrated designs of enclosures, cables, and connectors in the range of 55 dB or higher.
The above mentioned conventional solutions do not provide a high degree of shielding effectiveness and have high leakage problems (thus causing a loss of shielding effectiveness) and often require the use of manual assembly to apply the shields over the connectors and cables. Accordingly, what is needed are systems and methods which provide adequate EMI shielding to cables and connectors.
The present invention provides cables having a body that is surrounded by a vacuum metallized layer. The metallized layer can be grounded with a metallized thermoform connector to prevent the release or impingement of harmful EMI radiation.
Optionally, an insulating top coating can be disposed over the metallized layer over the cable body.
In one embodiment, the metallized layer is coupled to the ground with a conductive connector that is positioned on an end of the cable body. Exemplary conductive connectors of the present invention are typically composed of a metallized thermoform. The thermoform is either a one piece (i.e. clamshell) or two piece assembly. The thermoform can be sized to substantially conform to the shape of a pin connector assembly of the cable body. The metal layer on the thermoform is electrically coupled to an exposed portion of the metallized layer on the cable body by snap fitting the thermoform around the end of the cable with a tongue and groove assembly, press fit with a conductive epoxy or gasket, laser welded, or the like.
In some arrangements, the entire cable body is surrounded by the metallized thermoform to shield the conductors disposed within the cable. The thermoform will typically be thin walled or ribbed so as to allow flexing of the cable body. The metallized layer can be disposed along either an inner surface of the thermoform (so as to not require an insulating layer) or along the outside layer. If the metallized layer is disposed on the outside layer, there will typically be an insulating layer covering the metallized layer to prevent electrical contact with any surrounding electronic elements.
Metallization of the cable body and thermoform can be applied through vacuum deposition (i.e., cathode-sputtering, ion-beam, or thermal vaporization), painting, electroplating, electroless plating, zinc-arc spraying, or the like.
In exemplary embodiments, metallization of the cable body and of the thermoform is through a vacuum deposition process, which maintains a temperature of the cable body or thermoform typically below approximately 150xc2x0 F., and preferably below approximately 120xc2x0 F. during the manufacturing process. The low temperature vacuum deposition process can create a substantially uniform conductive layer without substantially warping or distort the underlying thermoform or dielectric. The evenly coated surfaces, creases, recesses, and edges of the thermoform create less impedance variations in the conductive layer and the overall shielding effectiveness of the shield can be improved.
The metallized layers of the present invention can theoretically provide attenuation levels between 0 dB and 110 dB, but typically between 20 dB and 70 dB. It should be appreciated, however, that it may be possible to provide higher attenuation levels by varying the thickness and material of the metallization layer.
To reduce the EMI leakage at the joint between the connector and cable body, the attachment surfaces of the metallized thermoform connector can include bumps, protrusions, or other blocking elements that reduce the size of the gaps to a size that is no larger than one half the wavelength of the target EMI/RFI radiation.
In one exemplary embodiment, the present invention provides a method of shielding a cable. The method includes providing conductive leads encapsulated within a dielectric layer. A metallized layer is applied over the dielectric layer. A metallized thermoform connection assembly can be electrically coupled to the metallized layer over the dielectric layer and a grounded housing. In exemplary methods, the metallized layers are thermally vaporized onto the dielectric layer and the thermoform so as to form a substantially uniform layer.
In some embodiments a base coating will be applied between the dielectric cladding (or polymer overcoat) and a vacuum metallized layer to improve adhesion. In most configurations an insulating top coating is applied over the metallized layer to prevent electrical contact of the metallized layer with adjacent electrical devices or components.
In another exemplary embodiment, the present invention provides a cable shield. The cable shield includes a thermoform body having an inner surface and an outer surface. A metal layer is applied to either the inner or outer surface. A cable body can be disposed within the thermoform shield. The cable shield can be grounded to provide EMI shielding for the cable body. The thermoform body can comprise a single xe2x80x9cclamshellxe2x80x9d piece or two separate bodies that can fit around the cable body. Optionally, the thermoform body can be ribbed so as to allow the cable body to flex and bend.
In some embodiments, the cable body and/or thermoform can be metallized over two surfaces. In addition to increasing attenuation of the impinging radiation by 10 dB to 20 dB, the second metallized layer provides insurance against the creation of a slot antenna. Thus, if one of the layer is scratched or otherwise damaged, the second metallized layer can still block the emission or impingement of the radiation.
For a further understanding of the nature and advantages of the invention, reference should be made to the following description taken in conjunction with the accompanying drawings.